Why does using flash seem to reduce motion blur at the same shutter speed?
Asked 2/24/2016
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When I shoot with my Canon 600D at a shutter speed that gives motion blur without flash, the same shutter speed often looks much sharper when I use the built-in flash. Why does flash seem to freeze motion even though the shutter speed hasn't changed?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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Simple: the flash duration is very short. A full-powered flash is usually around ¹⁄₂₀₀th of a second, and if less than full power is used, it can be in the ¹⁄₁₀₀₀₀th range. If this flash is the only significant contributor of light in the image (and indoors, it's easy for that to be the case), the rest of the time the shutter is open just doesn't matter. That short pulse is enough to freeze most motion.
On the other hand, if there is enough ambient light compared to the shutter duration that that does contribute meaningfully to the exposure, you may get a double image — a sharp image from the flash, plus ghosting. See What is "Dragging the Shutter"? and How does dragging the shutter work? for more on this, as well as When should you use a normal flash vs a second-curtain flash?
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
10y ago
0
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Flash can appear to reduce motion blur because the flash burst is much shorter than the shutter speed. A flash pulse may last around 1/200 s at full power, and much shorter at lower power, so if the flash provides most of the light, the subject is effectively lit for only that brief instant. That short burst freezes motion.
The shutter may still stay open longer, but if ambient light is weak, it contributes little to the exposure, so blur from the rest of the exposure is minimal. In that case, the image looks sharp because most of what you see was recorded during the flash pulse.
If ambient light is strong enough, you can still get motion blur or a ghosted/double image: a sharp flash-lit subject plus blurred ambient exposure. So flash does not magically change the shutter speed; it changes which light is doing most of the exposing.
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